by James Morris
In celebration of Valentine’s Day, let’s take a look at how we evolved to see the color red, and all of the other colors in the rainbow. It’s not as complex as you might think.
There is an old idea, going back to the time of Aristotle in ancient Greece, that all living things can be organized like rungs on a ladder, or links in an unbroken chain. There are lower forms, like microbes and plants, and higher forms, like animals and humans. Every species has its special, unchanging place in this hierarchy. This way of organizing life is sometimes known as the Chain of Being or Scale of Nature. It has persisted in various forms for about 2000 years, showing up not only in science, but also in literature, art, and language.
That is, until Charles Darwin upended this world-view. Darwin suggested that organisms are not fixed in one particular place – they change or evolve over time. He also saw the process of evolution not as a ladder of progress, but instead as a branching tree, with the trunk of the tree being a common ancestor, the broken limbs being extinct forms, and the tips of the branches living organisms, like bacteria or whales or humans.
In On the Origin of Species, Darwin writes, “As buds give rise by growth to fresh buds, and these, if vigorous, branch out and overtop on all sides many a feebler branch, so by generation I believe it has been with the great Tree of Life, which fills with its dead and broken branches the crust of the earth, and covers the surface with its ever branching and beautiful ramifications.”
This “view of life” means that it really does not make sense to talk of “lower” or “higher” forms. All organisms living today are the result of billions of years of evolution of life on this planet, and all are adapted, in different ways, to current environmental conditions. The bacteria living today are not the same as those that first lived about 4 billion years ago. They too have evolved over time.
And yet, this idea of low and high forms persists tenaciously. We often slip into language reflecting this ancient idea. When I lecture on biology or evolution, I inadvertently find myself saying, “from bacteria up to humans.” We humans typically see ourselves at the top of this ladder, the end of the line, the cat’s meow, the crowning achievement of evolution.
We are certainly complex in many ways. We are made up of trillions of cells. If number of cells corresponds with complexity, then we are enormously complex. Of course, a singled-cell organism could make the reverse argument, explaining that humans need trillions of cells to do what a single cell can do with just one (survive, reproduce, move about, etc.).
But a singled-celled organism can’t write, or think, or drive a car. Humans have billions of neurons and a brain capable of complex thoughts and even consciousness. We have language allowing us to convey all kinds of concrete and abstract ideas.
In spite of this complexity, there are many ways that we are simpler than other organisms. Bacteria, for example, have many ways of harnessing energy from the environment. We are stuck with one: We break down carbon-based molecules (like sugars, fats, and proteins) and derive energy from them.
Sean Nee, in his Nature article “The Great Chain of Being,” retells the familiar history of life on Earth from the perspective of these diverse bacteria, putting us in our metabolic place.
Another humbling reminder of our “place” in nature comes from taking a look, so to speak, at color vision. We see colors because of three types of cone cells located in the retina of our eyes. Each type of cone cell contains a different pigment that is sensitive to its own range of wavelengths of light, with peaks around red, green, and blue. We integrate input from these three types of cone cells to see the rainbow of colors that we do.
Sounds pretty complex. But what about other vertebrates? You might be surprised to learn that most fish, amphibians, reptiles, and birds don’t have three but actually four types of cone cells, and therefore see colors differently than we do.
How did this happen? During the evolution of early mammals, from which we are descended, not one but two pigment genes were lost from the four that their vertebrate ancestors had. Therefore, these mammals were left with just two types of cone cells. In other words, they were essentially colorblind.
It is thought that this change occurred because early mammals were nocturnal (active at night, not during the day). It’s likely that color vision was no longer an advantage to these mammals and was lost. Most mammals retain two-color vision even today. That is, cats and dogs are colorblind, as are mice, cows, and horses.
So how did we end up with three-color vision? In one group of primates – the Old World monkeys – one of the pigment genes, by chance, duplicated, resulting in two copies of the gene. One of these copies accumulated small changes (mutations) over time and became sensitive to a different range of wavelengths of light than the original pigment gene. This turned out to be an advantage as these primates took on a diurnal (active during the day) lifestyle and was useful for seeing colorful, ripe fruits.
We are descended from this group of primates, along with chimpanzees, gorillas, and orangutans. This is why we have three types of cone cells, more than other mammals, but fewer than many other vertebrates.
There are two interesting codas to this colorful story. Because two of our three pigment genes are located in the X chromosome and males have just one X chromosome, a defect in a pigment gene in males is always expressed. This is why a staggering 6-8% of males are colorblind. Colorblindness in females, with two X chromosomes, is much more rare since one normal copy of the gene is sufficient to see colors.
Mutations can sometimes render a pigment gene non-functional and lead to colorblindness. But mutations can also change the range of wavelengths that a particular pigment is sensitive to, as we saw in Old World monkeys. And because females have two X chromosomes, it is thought that some females actually have four different types of cone cells, rather than three, and have four-color vision.
Do you suppose that this ability places them on a higher rung of the ladder than the rest of us?
© James Morris and Science Whys, 2016.